1. Field of the Invention
The present invention relates to the preparation of cellulose fiber pulp from wood and other cellulosic materials. More particularly, the present invention relates to the flushing or washing of lignin, digestion and bleaching chemicals from cellulosic pulp.
2. Description of the Prior Art
Raw wood, bagasse and other cellulosic fiber sources are delignified by cooking processes in the presence of chemicals which form water soluble compounds and complexes with the natural lignin binder of the raw fiber matrix. Although the chemicals used in the digestive cooking process are relatively inexpensive, those quantities consumed in the 1500 tons of dry pulp per day production of an average pulp mill necessitate an economical recovery and recycle of such chemical values. Moreover, the lignin compounds which must be removed from the cellulose fiber matrix contain sufficient heat value and volatility to contribute favorably to the over-all mill heat balance.
The objectives of chemical and heat value recovery from wood cooking liquors are gained simultaneously in a pulp mill recovery furnace. Chemically hydrolyzed lignin, called black liquor, is water flushed from the pulp on a filter surface which permits the liquor and water to drain from the pulp while the fibers are supported and retained on the filter.
As washed from the pulp, black liquor contains approximately 10% to 20% solids in solution and suspension with water. To recover the heat and chemical values present in black liquor, the solids concentration of the solution must be increased to approximately 60%: sufficient to fuel a sustained combustion. This is normally accomplished by evaporation. The 60% solids heavy black liquor is burned in the recovery furnace to release both inorganic chemical values combined therewith and heat to generate steam. A portion of such liquor generated steam is used in a continuous evaporation flow stream for black liquor concentration with the remainder used in support of other mill processes such as paper drying.
This interrelated chemical recovery process is economically dependent on the balance between heat value and water in the black liquor flow stream. Excess water in the liquor stream adds to the heat demand for liquor evaporation thereby reducing the quantity of heat available from lignin fuel to support other mill processes. Such other mill processes must consequently be supported by purchased, supplemental fuels thereby adding dramatically to the over-all mill energy costs.
The usual source of such excess liquor water is at the pulp washers, the first objective of most pulp mills being a clean pulp. Excess lignin remaining in the pulp beyond the washers adds to the bleaching chemical costs or finally, in unacceptable paper quality.
From the foregoing, it should be appreciated that pulp washing efficiency is pivotal to the favorable economics of a pulp mill.
The currently predominate pulp washing technique includes the use of two to seven rotary drum vacuum filters such as described by U.S. Pat. Nos. 3,363,744; 3,403,786; 3,409,139 and 4,138,313. Pursuant to this practice, a slowly rotating drum filter is partially submerged in a mixing vat containing a 1% to 3% consistency slurry of pulp. A partially evacuated drum interior draws the slurry against the submerged, filter screen surface of the drum. Pulp fibers are retained on the screen surface while a portion of the water contained in the pulp passes therethrough. Such fiber accumulation on the drum screen surface builds a fiber mat thereon until drum rotation carries the mat above the mixing vat slurry surface. In an arcuate increment between emergence from the vat surface and reentry into the respective wash stage vat, the mat is peeled from the drum surface and directed into the vat of the subsequent wash stage where the process is repeated.
An alternative pulp washing technique in minority usage relies upon an endless horizontal belt filter similar to a fourdrinier papermaking screen. U.S. Pat. Nos. 4,154,644; 4,046,621; 3,939,077; 3,454,970 and 3,426,908 include representative descriptions of horizontal belt washers. The slurried pulp is deposited in a continuous flow stream onto the traveling belt topside which retains and supports the fiber mat residue of pulp fiber. On the underside of the top run of the belt is provided one or more vacuum pits to generate an atmospheric pressure differential across the belt and the mat deposited thereon.
Both drum washing and belt washing techniques rely upon a plurality of wash stages wherein the fiber is flushed with wash liquor and drained in each stage. As the pulp washing sequence advances from the first to last stage, filtrate drawn from the pulp advances counterflow of the pulp so that the filtrate of each stage is used to wash the preceding stage pulp mat.
In theory, plug flow displacement of mat liquor with more dilute wash liquor provides the least mixing of the respective liquors and the greatest wash efficiency. If ideal plug flow was attainable in all wash stages, no more fresh water would be added to the last stage than is discharged with the pulp from the last stage. Unfortunately, the ideal is not attainable in practice due to the fact that the fibrous mat is neither homogeneously permeable nor porous. Mat liquor contained within the interstitial matrix between the fibers is not uniformly available to wash liquor displacement. Accordingly, the available wash liquor passes through the mat along a dispersed system of channels and interconnected large pores. These channels and interconnected large pores are flushed of mat liquor but relatively large quantities of additional liquor trapped in closed or restricted pores remains to be carried over into the next wash stage.
From the foregoing understanding of mat washing mechanics, a staged series of drum washers has an inherent efficiency advantage over the same number of stages in a single belt washer due to reslurrying of the fiber and mat reformation respective to each drum stage. For each drum wash stage, a new mat is formed along with a new set of flow channels respective to each new mat. Similarly, each time the pulp is reslurried, liquor trapped in the closed pores of the previous mat is released and diluted. Conversely, the capital cost of each stage of drum washing equipment is considerably greater than the per stage cost of belt washing equipment.
It is, therefore, an objective of the present invention to provide a method and apparatus for improving the washing efficiency of pulp on belt filters over that previously attainable by the prior art.
Another object of the present invention is to improve the washing efficiency of a belt washing unit by improving the efficiency of each washer stage along the belt run.
Another object of the present invention is to increase the percentage of interstitial mat liquor present in a belt filter mat that is available to wash liquor displacement.
Another object of the present invention is to teach a wash liquor application sequence that removes a greater percentage of interstitial liquor present in a belted filter mat with no more than a prior art quantity of wash liquor.